This invention relates to a filtering device and more particularly to a filtering device which comprises a cylindrical member bored with a large number of holes, and a filter element surrounding said cylindrical member, whereby any liquid filtered by the filter element can smoothly flow into the cylindrical member.
With some filtering device of the above-mentioned type, the filter element is folded like bellows to provide an increased area of filtration and rolled to surround a cylindrical member so as to cause the folded portions of said filter member to project in the radial direction of the cylindrical member. A filter element constituting the most important component of the filtering device is a porous flexible sheet generally prepared as is well known, from vegetable fibers such as those of cotton, chemical fibers such as polyester, glass fibers, soft foamed plastics material having continuous cells, or asbestos. FIG. 1 illustrates a known filtering device using such filter element. The prior art filtering device comprises a cylindrical member 11 bored with a large number of holes 10 (only some of which are shown) and a crosswise folded filter element 12 so rolled as to concentrically surround the cylindrical member 11. Numerous folded portions 13 of the filter element 12 prepared from any of the aforesaid materials project outwardly in the radial direction of the cylindrical member 11. Each projecting folded portion of the filter element 12 has a pair of oppositely inclined side walls 14 which jointly define an interior space 15 having a substantially triangular cross section. While passing from the outside of the projecting folded portion 13 into the interior thereof, a liquid is filtered, and the filtrate passes through the interior space 15 and holes 10 into the cylindrical member 11. While duct contained in the liquid is little deposited on the filter element 12 in the initial stage of filtration, liquid pressure is uniformly applied to the entire outer wall of the filter element 12, enabling the projecting folded portion 13 of the filter element 12 to be left open as shown in FIG. 1, and in consequence the liquid to pass through the whole filtration surface. Where, however, deposition of dust in the root region 16 defined between every adjacent projecting folded portions 13 leads to unbalanced liquid pressure (this event takes place relatively early during filtration), then the mutually facing side walls 14 of the projecting folded portion 13 are tightly pressed against each other with the resultant loss of the interior space 15. As the result, the fibers of one side wall 14 tightly interdigitate with those of the other side wall 14 to close up filtering spaces between the respective fibers, substantially obstructing the filtration of the liquid through the mutually pressed side walls 14 of the projecting folded portion 13, said filtration being only allowed in the root region 16. However, the root region 16 having an extremely smaller filtration area than the entire outer surface of the filter element 12 is quickly coated with dust. Once such an undesirable event as shown in FIG. 2 arises, a sharp decline occurs in the filtering capacity of a filtering device, giving rise to a considerable loss of pressure therein. With the prior art filtering device of the above-mentioned type, therefore, a plugged filter element must be exchanged for a fresh one in a short time.
To eliminate the drawbacks of the conventional filtering device, there have been proposed various methods of, for example, reinforcing the filter element with plastics material, or folding both filter element and a wire net together. These prior art methods proposed for resolution of difficulties encountered in the past have a common characteristic of rendering the filter element stiff or mechanically strong, but are still accompanied with the drawbacks that the filter element is manufactured by a complicated process at high cost and moreover has to be folded at fewer points than otherwise, namely, is unavoidably decreased in filtering area.